The present invention relates to a hydraulic braking pressure control system in which an output chamber and a control chamber are defined in a housing with opposite ends of a control piston facing them, respectively. The output chamber leads to a brake device, while the control chamber is connected to a control hydraulic pressure source and a reservoir through a switchover valve means capable of changing over the connection and disconnection. A cut-off valve is interposed between an input chamber defined in the housing and connected to a master cylinder and the output chamber, and is adapted to be closed in response to the movement of the control piston toward the control chamber.
Such hydraulic braking pressure control systems are conventionally known, for example, from Japanese Patent Application Laid-open No. 222,354/85.
In such a hydraulic braking pressure control system, it is necessary to design the system to operate even if the control hydraulic pressure from the control hydraulic pressure source is abnormally reduced by any reason. In the above prior art system, it is possible to open the cut-off valve when control hydraulic pressure is abnormally reduced, thereby applying the hydraulic braking pressure from the master cylinder to the brake device.
However, if the control piston is permitted to be moved in a direction to reduce the volume of the control chamber, i.e., in a direction to increase the volume of the output chamber after control hydraulic pressure in the pressure source is abnormally reduced, it is difficult to increase the hydraulic braking pressure applied to the brake device. Therefore, Japanese Patent Application Laid-open No. 107435/85 discloses a hydraulic braking pressure control system in which the control chamber is tightly closed when control hydraulic pressure is abnormally reduced. In this system, however, when control hydraulic pressure is abnormally reduced, the cut-off valve is permitted to be closed, and a passage connecting the input chamber and the output chamber to bypass the cut-off valve is opened. Therefore, such a passage and a means for opening and closing the passage are required in addition to the cut-off valve, resulting in a complicated construction.